Automatic transmissions known from practice are often configured with what is known as a fixed displacement pump for hydraulic supply, the delivery volume of which fixed displacement pump is proportional to an input speed. This means that the delivery volume of the fixed displacement pumps increases continuously as the input speed rises. Here, the fixed displacement pumps are usually operatively connected via a transmission input shaft to a drive machine of a vehicle drive train, and are therefore driven proportionally with respect to the input speed of the drive machine.
In order to avoid undersupplied operating states of the transmission over the entire operating range of a transmission, at least in the range of the transmission actuator system, the delivery capacity of fixed displacement pumps is designed in each case for that operating point of the transmission at which the maximum hydraulic fluid volume requirement is requested by the transmission actuator system and, at the same time, the hydraulic pump is driven by the drive machine at the minimum input speed which, in the case of a configuration of the drive machine as an internal combustion engine, corresponds to its idle rotational speed.
Said design criterion disadvantageously leads to the pump delivering considerably more hydraulic fluid volume than is necessary for the supply of an automatic transmission in operating states of the vehicle drive train in which the drive machine is operated in speed ranges above the idle rotational speed. This circumstance undesirably impairs an efficiency of the transmission, which is ultimately reflected in an increased fuel consumption of the drive machine.
In order to improve an efficiency of an automatic transmission, automatic transmissions are increasingly equipped with variable capacity hydraulic pumps, the delivery volumes of which are variable in a regulated manner independently of the input speed. By way of controllable variable capacity pumps of this type, there is now the possibility to avoid the above-described undersupplied operating states of an automatic transmission, and to be able to operate an automatic transmission at as high an efficiency as possible by way of delivery volumes which are adapted in each case to the currently prevailing hydraulic fluid volume requirement.
DE 10 2014 207 798 A1 has disclosed a transmission which is configured with a variable capacity pump. Besides the variable capacity pump, the transmission includes a further hydraulic pump which is configured as a constant-capacity pump. During operation of a transmission, a hydraulic fluid volume flow which is to be fed to the transmission components is determined, and the delivery capacity of the hydraulic pumps is set in a manner adapted to the level of the hydraulic fluid volume flow. The hydraulic fluid volume flow, which is provided by the hydraulic pumps for supplying the transmission components, is divided between the transmission components in a manner dependent on the operating state, such that the variable delivery capacity of the hydraulic pump can be set in each case to a minimum in order to implement operation which is as optimized as possible in terms of efficiency.
The variable capacity pump is the main transmission pump which is coupled to a transmission input shaft and can therefore be driven by a drive machine connectable to the transmission input shaft of the transmission. In contrast to this, the second pump unit is coupled to the transmission output shaft which in turn is connectable to an output of a vehicle drive train which is configured with the transmission, and is drivable at an input speed which is equivalent to the output speed. A pressure side of the variable capacity pump is connected to a primary pressure circuit in which the system pressure prevails, the system pressure in turn can be set via a pilot controllable system pressure valve. A pilot controllable valve device, known as a cooling valve, is provided downstream of the system pressure valve.
In addition, the cooling valve is assigned a thermal bypass valve, which is configured with a temperature probe, in order for it to be possible for a hydraulic fluid volume flow guided via a cooler to be set to the desired extent in a manner dependent on a currently prevailing operating temperature.